The field of endomicroscopy has emerged over the past decade with several innovations enabling microscopic imaging of living tissues of humans and animals and other subjects without the previously required removal of tissue for physical sectioning into thin slices for examination under a bench microscope.
Various forms of optical sectioning microscopy have featured, including laser scanning confocal microscopy (us, MKT), multiphoton microscopy and other forms of non-linear scanning microscopy.
Clinical evidence has amassed as to the clinical benefit of endomicroscopy, including increased sensitivity and specificity of disease detection and, if used to select sites for traditional biopsy sampling, achievement of higher diagnostic yield from a reduced number of biopsy sampling sites.
The most established systems in practical clinical use have been used most in gastroenterological endoscopy (GI endoscopy) and involve two main technical approaches to miniaturization of the confocal microscope sufficient for endoscopic use. Both these approaches involve the use of optical fibre technology to provide a flexible conduit that separates bulky laser source and detection components from the imaging components near the tissue.
Of the two commercially available instruments, the first uses a single optical fibre acting as both an illumination and detection aperture, and the fibre is physically scanned in a raster pattern and this occurs within the distal end of the device. A miniature objective lens couples the plane traversed by the scanning fibre tip to an objective imaging plane at or beneath the surface of the tissue in front of the device. This lens is used bidirectionally, forward coupling the illumination source from fibre to sample, and then collecting fluorescent or reflected light from the focal plane and projecting it back into the source optical fibre form transmission back to the proximal detection unit. A contact window at the tip of the device in front of said lens provides a reference plane of tissue contact, and an actuation mechanism included in the imaging head moves the focal plane of the optical system to different distances beyond the contact window (thus effecting optical sectioning at dynamically variable depth relative to the surface of the tissue contacting the window), either by moving part of the optical system to shift the focal plane, or by moving the whole scanning mechanism and lens system relative to the window. This has resulted in devices that yield subcellular, sub-micron resolution across a usable sub-millimeter field of view, with dynamic adjustment of imaging depth under control of the endoscopist.
Commercially available scanners exploiting this approach have reached dimensions of 5 mm×43 mm as a rigid tip connected via a flexible umbilicus for integration with flexible and rigid endoscope devices. This is sufficiently small to allow integration into a modified gastrointestinal (GI) endoscope or surgical endoscope, the former requiring a slight rigidisation of a short length of the distal region of the endoscope to accommodate the rigid scanner components.
Variations on this approach have been proposed which include alternative scanning patterns (spiral fibre scanning, elliptical fibre scanning, lissajous pattern fibre scanning, and MEMS based mirror scanning) but at the dimensions required for endoscopy, none have yet been reported to produce the imaging performance or mechanical viability of the above approach.
The other of the commercially available instruments involves proximal scanning of a coherent imaging bundle of optical fibres, each acting in turn as a confocal illumination and detection aperture, sequentially. This approach removes the requirement for several of the moving parts required in the scanning fibre approach, and has facilitated more extreme miniaturization than the scanning fibre approach to date, allowing probes less than 3 mm diameter down to sub millimeter devices. This comes at the expense of resolution, being limited by the packing density of fibres (which cannot approach the resel density of a continuously moving fibre core), and devices in use have a maximum fibre count of 30,000 elements at the larger diameters down to less than 10,000 pixels for the smaller devices (compared to a 1024×1024 pixels, or 1 megapixel, associated with the scanned fibre devices). These devices also do not allow the dynamic adjustment of imaging depth, although lens systems have been developed which fix individual devices at specific imaging depths, which may be at or beneath the tissue surface (see MKT specifications). The great advantage of these probes is that they are small enough, and have a sufficiently short rigid length at the tip, and are sufficiently flexible in between, to allow insertion through working channels of existing endoscopes without modification or integration.
To date, this has not been possible with the scanning fibre devices, due in part to diameter, but mostly to the rigid length resulting in the longitudinal arrangement of window, then objective lens, then fibre scanning region, fibre mount, and depth actuation mechanism.
Although further miniaturization of the scanned fibre approach has been demonstrated, the rigid length of the scanner, while shortened, still precludes insertion via working channels of unmodified GI endoscopes and still require integration into the endoscope. Sufficient further shortening of the rigid length of said scanner designs is limited little by the ability to make smaller components but fundamentally by the physics dictating the required longitudinal arrangements. Although the diameter of the components in the above have approached the diameter required for insertion into flexible endoscope working channels, the rigid length remains too long to navigate the bends in the device, including the permanently angled insertion port typically configured to ensure that if a device can be inserted into the first part of the channel, it will subsequently be able to negotiate any path through the endoscope in normal use.